The capacity to form organic S was determined for microorganisms present in soil samples, collected from different depths of an eastern white pine forest. Inhibitors of eukaryotes (cycloheximide and amphotericin B), gram-negative prokaryotes (polymyxin B), and aerobically-respiring prokaryotes and eukaryotes (sodium azide) were used to assess the relative contribution of each group to organosulfur formation over a wide range of added sulfate. Values for microbial biomass (direct counts and ATP content) and activity (soil respiration) were estimated and correlated with organosulfur formation potentials at various soil depths. Most of this latter activity, regardless of concentration of added sulfate, was mediated by aerobically respiring prokaryotes. In each horizon, however, increasing concentrations of sulfate induced a shift in the physiological types of microbial populations responsible for organic S formation. This is consistent with the observed multiphasic uptake kinetics for sulfate characteristic of samples collected from each horizon. ATP pool sizes and native soil respiration rates exhibited positive relationships with organosulfur formation with 7.5 nmol sulfate (r = 0.86, 0.81, respectively) and saturating concentrations of sulfate (r = 0.79, 0.72, respectively) were employed, separately. Amounts of added sulfate also had a positive effect on soil respiration rates (P lt 0.0001). Collectively, the data suggest that exposure to sulfate stimulated endogenous aerobic respiration, generating ATP, that was used in part to form organic S.